EUV light, which is defined as electromagnetic radiation with wavelengths between 124 nm and 10 nm, is used in next-generation photolithography systems to produce structures smaller than is possible with current ultra-violet light sources, such as excimer lasers. However, EUV light is more difficult to generate than the ultra-violet light used in current photolithography systems. EUV light sources for photolithography systems commonly use a laser to transform xenon or tin into plasma, which plasma then emits radiation in the EUV portion of electromagnetic spectrum.
There are several disadvantages associated with this type of laser produced plasma EUV light source. In order to have sufficient photolithographic throughput, the EUV light source must have high average power (100 W or above). However, to achieve this level of EUV light output, the driver laser used to generate the plasma must have an output of greater than 10 kW. Given the typical efficiencies of the solid state lasers used as driver lasers in EUV light generation, a laser produced plasma EUV light source with an output of 100 W may use in excess of 200 kW “at the wall.” As the predominant commercial use for EUV light sources is in photolithography systems, most EUV light sources are designed to have high EUV light output (100 W or above) and thus have a correspondingly enormous energy requirement. Further, given the high costs associated with building EUV light sources and the lack of low-power alternatives, even in applications in which high output EUV light is not required, EUV light sources with high output are used.
Additionally, such laser produced plasma EUV light sources can use tin or other metals as the driver laser target material. The process of converting the metal target to plasma creates both micro-particles of the original target and metal vapor after the plasma has cooled. Both of these byproducts foul the optics and vacuum chamber of the EUV light source, necessitating difficult and costly maintenance to remove the metal deposits.
Even in laser produced plasma EUV light sources that use xenon or other gases as the driver laser target, not all of the target gas that is injected into the vacuum chamber, usually in the form of cryogenic droplets, is successfully converted to plasma. The droplets may not intersect with the driver laser pulse, as in the case when the droplet injection stream is inaccurately timed or aimed. Also, to increase the odds that a driver laser pulse always strikes a droplet, droplets may be injected into the vacuum chamber at a higher rate than can be reacted upon by the driver laser. This over supply of droplets may also result from the method of droplet generation, which relies on stimulation of a gas supply nozzle at the natural frequency associated with the gas. As EUV light is strongly absorbed by most materials, including gases, the excess gas injected into the vacuum chamber only serves to attenuate the EUV light that is generated.
Thus, what is needed is a laser produced plasma EUV light source for applications that do not require high EUV light output that does not have the high operating costs of high-power EUV light sources and that efficiently use the driver laser target material so as not to attenuate the generated EUV light or require frequent costly maintenance.